This invention relates to artificial biocompatible synthetic vertebral prostheses, and more particularly to prosthetic metal intervertebral discs.
Many types of vertebral prostheses have been proposed and patented for implantation in the vertebral disc space after surgical removal of a diseased or damaged disc. Such devices fall into three broad categories. One category of prostheses includes the use of pliable synthetic materials in an attempt to mimic the compressibility of the human spinal disc. For example, U.S. Pat. No. 5,171,281 (Parsons) discusses a disc spacer which purports to possess mechanical properties akin to those of the normal disc by varying the hardness of the elastomeric material in its nucleus and annulus. U.S. Pat. No. 5,192,326 (Bao) illustrates a prosthetic disc formed from a multiplicity of hydrogel beads having a water content of at least 30%. According to the patent, a semi-permeable membrane covers the beads and is said to permit fluids to flow in and out of the prosthetic nucleus. U.S. Pat. No. 5,071,437 (Steffee) proposes another approach to a pliable implant. That approach appears to involve upper and lower flat rigid plates sandwiching an elastomeric core made from a polyolefin rubber. U.S. Pat. No. 5,002,576 (Fuhrmann) also discusses a variation on the foregoing approach.
Another type of prosthesis attempts to mimic the shape of a natural spinal disc. For example, U.S. Pat. No. 4,714,469 (Kenna) discusses a spinal implant adapted to replace a disc between adjacent vertebrae using a predetermined thickness and profile to match the space between the vertebrae. The implant includes a porous coating on its surface. U.S. Pat. No. 4,759,766 (Buettner-Janz) illustrates a metallic disc endoprosthesis which has two symmetrical, concave end plates with an intermediate spacing piece. Similarly, U.S. Pat. No. 5,314,477 (Oka) discusses a disc prosthesis including two plates separated by a joint composed of a spherical cap and cylindrical base which attempts to ensure connection between the vertebrae.
More recently, the present inventor has patented (U.S. Pat. No. 5,865,845, Thalgott) a prosthetic intervertebral disc which takes into consideration the anatomy and the geometry of the intervertebral space sought to be filled by the prosthetic device. That device, while an improvement over prior available devices, does not work for all applications or geometries. Further, continuing experience with its use has increased understanding of areas where its design could be improved to mesh more precisely with the surrounding anatomy in the spine. Accordingly, a need continues to exist for an improved intervertebral implant which better addresses the need for a disc replacement which improves over prior spinal implants.
The foregoing objects are achieved and the disadvantages of the prior art are overcome by providing a spinal disc implant comprising an implant of a biocompatible metallic body having a front wall, a rear wall, and two side walls extending therebetween to define an interior space. Each side wall defines a substantially arcuate curve joining one end of the front wall and one end of the rear wall. Further, each wall contains at least one opening therein, and the upper and lower surfaces of the two side walls having a plurality of teeth extending therefrom for engaging adjacent vertebral bodies. The side walls of the implant taper in height from the rear wall to the front wall; and the front wall and the rear wall each lack teeth, and have a height substantially similar to that of a portion of the side wall adjacent the front and rear walls. The implant is made of a biocompatible metal such as titanium or an alloy thereof, and the first and second sides taper from the junction between the back wall and the side wall to the junction between the front wall and the side wall. Preferably, the interior space includes a porous hydroxyapatite block shaped and press fit, cemented or screwed in to fill the interior space. The porous hydroxyapatite substance helps the prosthesis integrate into the vertebral structure by allowing bone to grow into pores.